Soldering joins copper with a torch and filler metal. A press-fit fitting is crimped by a tool around a rubber seal, with no flame. At a water heater, heat must stay off the tank's own fittings. A. O. Smith says to solder pipe to the adapter first. Press-fit adds no joining heat.
What do "soldered" and "press-fit" actually mean here?
Soldered joints are made by heating the copper and fitting with a torch until filler metal flows into the gap. Press-fit joints use a fitting with a rubber sealing ring inside; a powered tool squeezes (presses) the fitting onto the pipe. Viega, a major press-fitting maker, advertises that press connections can be made "wet or dry in seven seconds or less" and describes its press line as "No hot work."
Either style can be a correct, permanent way to join copper water pipe. The difference is the process around the joint: open flame and fire protection for solder, tool access and tube preparation for a press. Our page on press fittings versus solder for copper repairs covers that general choice. This page is about what changes when the pipe is next to a water heater.
Why does the tank's own fitting change the answer?
A. O. Smith's residential gas heater instructions put it this way: "Heat must not be applied to the water fittings on the heater as they may contain nonmetallic parts. If solder connections are used, solder the pipe to the adapter before attaching the adapter to the hot and cold water fittings." The same manual warns that some models "may contain energy saving heat traps to minimize the migration of heat to the pipes," and says "Do not remove the inserts within the heat traps."
That rule sets the sequence for a soldered job. The plumber builds the joint on the bench or in the open, lets it cool, and only then threads the finished piece onto the tank. Sweating a joint while it is already screwed to the heater can cook what is inside. A press-fit connection avoids the question on the pipe side, since pressing adds no joining heat. The threaded connection to the tank is still made the same way either way, with a good joint compound on tight fittings.
Is a press-fit joint rated for hot water?
For hot and cold potable water, yes, where the product is approved for it. Viega's application chart lists "Hot and cold potable water" for its copper press fittings with an EPDM seal, rated 0°F to 250°F (EPDM seal, per note 4 of the chart). That is the fitting maker's own rating. Check the exact fitting and seal in your installation against the maker's chart, because ratings belong to the product, not to press fittings in general.
What about solder itself?
The heater maker is specific here too: "Use only 95/5 tin-antimony or other equivalent solder. Any lead based solder must not be used."
Which should you choose?
| Question | Soldered (sweat) | Press-fit |
|---|---|---|
| Open flame near the heater, gas line and nearby framing | Yes, with fire protection | No joining heat |
| Heat near the tank's plastic and rubber parts | Must be kept away; solder pipe to the adapter first | None from the joint |
| Tools | Torch, flux, solder | Compatible press tool and jaws |
| Seal | Metal filler | Rubber ring (EPDM or other) rated for the use |
| Tight spots | Needs room for the torch and heat shielding | Needs room for the tool and jaws |
In a garage, closet or attic with a gas heater and stored items nearby, avoiding an open flame has real appeal. In a wide-open install, a skilled plumber may reasonably choose soldering. Neither is a shortcut around the other steps: connectors, shutoffs, the relief valve discharge, expansion control and the permit. For what connects the pipe to the tank, see which flexible connector to use at a water heater and what a dielectric union is.
What should you ask the installer?
- Which joining method are you using, and why for this location?
- Will the tank's fittings stay free of torch heat, with joints made on the adapter first?
- Is the press fitting approved for hot potable water, with a seal rated for it?
- Is the work permitted? Our Phoenix water heater permit page lists what the city expects.
If a joint weeps after the install, do not tighten or reheat it yourself. Shut the water off and call. HQ Plumbing and Air is licensed and insured, gives free estimates with upfront pricing, and handles water heater installs on tank and tankless units; see our water heater service.